Pulverising mills are often used for grinding geological mineral ore samples into a fine powder for analysis of their mineral content. A typical pulverising mill comprises: a platform which is driven in a vibratory and oscillatory manner to produce a generally planar orbital motion; and, a grinding head which is held on the platform.
Grinding heads generally comprise a metal grinding bowl, a lid and grinding member in the form of either a freely movable grinding disc or an annular ring along with a solid cylinder within the ring or a set of rings. The bowl is placed or affixed on the platform and a mineral sample (charge) together with the disc or ring set placed within the bowl. The lid is then secured to the bowl.
When the mill is operated, the disc or ring set are effected by the movement of the platform and under centrifugal force orbit and roll against the internal side wall of the bowl, such that the sample is crushed and ground between the disc or ring set and bowl wall to a very fine particle size. Examples of pulverising mills and grinding heads are described in the specifications of Australian patent Nos 570814; 585751; 569895 and 594525.
The bowl is constructed in several different ways dependent on its volumetric capacity. When dealing with small volumetric samples, say less than 2 kg of ore, the bowl, disc, lid and sample can be manually removed from the pulverising mill after each grinding cycle. to be cleaned and replenished with the next sample or charge.
These bowls generally have a hardened, wear resistant, metallic, cylindrical side wall and an integrally formed base. They are clamped to the machine platform by way of a fast release clamp such as an air bellow mounted to a rigid overhead cross arm which moves in unison with the platform or a cam lock device or other means extending up from the platform. After each and every sample, the entire bowl and contents are removed from the pulverising mill for emptying, cleaning and re-loading of the next sample.
When volumetric size of the sample charge becomes greater, the physical size of the bowl must also be increased. These larger grinding bowls are generally fixed to the pulverising mill platform by a bolt-down flange mounting which is formed as an integral part of the bowl side wall. These bowls are generally removed from the pulverising mill platform only when the bowl requires replacement due to wear, which may be after many thousands of samples have been ground. The grinding bowls are constructed of expensive high grade, abrasive resistant steel and consist of generally a cylindrical side wall or skirt, a base (either being integral with or detachable from the side wall) and, in larger capacity bowls a mounting flange which is constructed as an integral part of the side wall or skirt. It is the bowl skirt or side-wall that causes high production costs due to the volume of high grade material required in its manufacture from one solid billet of steel.
Another deficiency in currently available grinding heads, in many laboratories around the world, is the need for the operator to physically lift the grinding disc or ring set from the bowl after every grinding cycle, as part of the process for sample collection and cleaning to eliminate any carry over sample that may contaminate the next sample. With the single discs sometimes weighing over 20 kg it becomes very draining on the operator to continually carry out this task. The ring set (in particular a set of three rings) reduces the strain of any individual lift by distributing the mass of the grinding media across three various size individual rings. Though in production terms this is a much slower process due to the number of items that have to be handled and cleaned after every sample grind.
Another deficiency in currently available grinding heads, which use ring sets, is the concentration of wear in the form of deep concentric grooves in the base of the bowl which results in premature failure of the base and low grinding efficiency. Analysis of the wear pattern indicates that the orbital path of the grinding rings whilst in motion is not of the intended horizontal, rolling, sliding pattern. Rather, after an initial somewhat random grinding motion, the rings settle very quickly into a rolling harmonic pattern with the inner wall of the bowl and the smaller rings within each other. One quadrant of the large ring presses down on the base plate with the diagonally opposite quadrant on its upper edge is in rolling contact with the under side of the lid. Therefore, the largest ring operates inclined to the horizontal base of the bowl.
The inner grinding ring rolls in harmony with the outer ring and because it derives its motion from the contact pressure against the inner diameter of the larger outer ring it tends to follow the angle of the large ring and thus it too rolls against the lid with only a point contact left in touch with the base of the bowl. This action greatly reduced the contact work area between the rings and the bowl, leading to a greatly reduced efficiency and excessive premature wear of the bowl base and lid.
Another deficiency in grinding heads that use a bowl and freely movable disc and or ring set is the need to use an overhead arm, which is attached to the moving platform, to clamp the lid firmly onto the bowl during the grinding cycle. The extra mass of the overhead arm effects the orbit pattern of the spring mounted platform due to its unbalanced arrangement.
Various aspects of the present invention were developed to address one or more of the above noted deficiencies in the prior art.